text:
NOTAM D item spanning across midnight with EXC
The specification of NOTAM is somewhat flaky. Edge cases may even be hard to parse programmatically unless EUROCONTROL guidelines are being followed as well.
For the sake of simplicity, let's assume the following examples are valid from 2022-01-01 thru 2022-01-31.
D items (which specify the exact validity periods of the NOTAM) may span across midnight as follows:
D) 01-10 2100-0600

This is to be interpreted as follows:

Valid for ten nights beginning with 2022-01-01 21:00 thru 2022-01-02 06:00 and ending with 2022-01-10 21:00 thru 2022-01-11.

So even though the dates mention only 01-10, January 11 is also included as the times span across midnight. So far, so easy.
Things get hairy once exceptions are added to the mix:
D) 01-10 2100-0600 EXC WED

The only Wednesday in that period is on 2022-01-05. The above can be interpreted in two ways:

On exactly 2022-01-05, the times do not apply. So the NOTAM is not valid that Wednesday from midnight to 06:00 and from 21:00 to midnight.
The entire period starting 2022-01-05 is excluded. So the NOTAM is not valid that Wednesday from 21:00 thru the following Thursday 06:00.

I tend towards interpretation 2 since the other interpretation produces somewhat fragmented validities and I'm having a hard time imagining a situation where this actually makes sense.
But I can't find any hint about this case in the documentations. Has anybody seen such a case in the wild? Or maybe some more detailed official guide as of how to interpret such cases?
Thanks a lot!

I'm a former Brazilian NOF (NOTAM Office) operator. The Brazilian regulation for NOTAM is ICA 53-1 (click on Download do arquivo, open the downloaded PDF file and then see Exemplo 9 at page 42/60), which is compliant with ICAO's DOC 8126 (Chapter 6 - NOTAM) guidelines.
Based on DOC 8126 (Chapter 6) + ICA 53-1 + my experience as NOF operator, on the context described above, if I want to communicate circumstance #1 (i.e. the NOTAM is valid except on Wednesday 00:00 until 06:00 and 21:00 until 23:59), then I must type item D this way:
D) 01-10 2100-0600 EXC WED

(because EXC WED is a short for EXC WED 0000-2359, i.e. it's the whole Wednesday)
...or, alternatively:
D) 01-10 (EXC WED) 2100-0600

...even though the most appropriate way to express it more clearly (it's the one that I would use) is this:
D) 01-03 07-10 2100-0600 04 2100-2359 06 0000-0600 2100-0600

On the other hand, if I want to communicate circumstance #2 (i.e. the NOTAM is valid from Tuesday 21:00 until Wednesday 06:00 but not from Wednesday 21:00 until Thursday 06:00), then I must type item D this way:
D) 01-10 2100-0600 EXC WED/THU

...or, alternatively:
D) 01-10 (EXC WED/THU) 2100-0600

...even though the most appropriate way to express it more clearly (it's the one that I would use) is this:
D) 01-04 06-10 2100-0600

If I want the NOTAM to be valid except from Tuesday 21:00 until Thursday 06:00, then the item D becomes this:
D) 01-10 2100-0600 EXC TUE 2100 TIL THU 0600

...even though the most appropriate way to express it more clearly (it's the one that I would use) is this:
D) 01-03 06-10 2100-0600

In a nutshell:

WED means the whole Wednesday, while WED/THU means "from Wednesday to Thursday"; and
NOTAM operators are expected to use number notation instead of abbreviated week day names, precisely because of the confusion caused by these abbreviated names. Usage of abbreviated week day names is only justified when longer periods make number notation impracticable, e.g. 01-14 2100-0600 EXC WED is preferable because number notation would require the text 01-03 07-10 14-15 2100-0600 04 11 2100-2359 06 13 0000-0600 2100-0600 even though the general rule of thumb is that NOTAMs must be short (but also clear i.e. easy to understand: they can't be dubious).

Thanks a lot for this extensive answer. Just a little followup: I've never actually sen the WED/THU notation. Single days of month are separated by spaces 1 5 8, a range of days of month use the dash 5-7. How come days of week use the slash instead? Is this documented somewhere?

@svoop You're welcome. I recall seeing WED/THU, but it's somewhat rare (precisely because number notation is overall preferred). Days of month are indeed separated by spaces, provided that they're not a range, as you mentioned, i.e. 01 03 05 is correct but not 01 02 03 04 05 (instead, the correct is 01-05). The / (slash) indicates a change of day, thus WED/THU means the event starts one day, then the day changes but the event's still ongoing. I don't have access to ICAO's DOC 8126 anymore, but Brazil's ICA 53-1 features this example at page 42/60: MON/TUE TIL FRI/SAT 2000-0400.

DOC 8126 is available online here https://www.icao.int/NACC/Documents/eDOCS/AIM/8126_unedited_en%20Jul2021.pdf and NOTAM D-items are described on page III-6-14. But it's really minimalistic and doesn't say much about edge cases, also the WED/THU notation is not mentioned. (No surprise though, multiple schedules on one D-item are ignored as well why some NOTAM offices don't use commas to separate those as encouraged by EUROCONTROL as of https://www.eurocontrol.int/sites/default/files/2021-07/eurocontrol-guidelines-opadd-ed4-1.pdf

@svoop Thanks, I wasn't aware of Eurocontrol's mandatory guideline forbidding the use slash/oblique in Item D. Weirdly enough, ICAO's DOC 8126 is currently omissive about whether slash/oblique can be used or not. This is the likely reason why Eurocontrol forbids slash but not e.g. Brazil. For instance, see Brazil's international NOTAMs O0501/22 and I0925/22.

NOTAM really are an epic source of fun.

@svoop Haha, true.


text:
Delta plane looked like it was landing on Atlanta freeway
Yesterday a few minutes after 4:00pm ET (2020-01-04 21:00 UTC) I drove my car out of Park-Ride Lot C of Hartsfield-Jackson Atlanta International Airport in Atlanta and took 85 South to 285 West/North.

When I reached the 285 N part of my drive, a southbound airliner was descending straight toward me, and since Hartsfield doesn't have north-south runways, I immediately felt like something was "off".
Having lived in San Diego for many years (which has an airport VERY close to the city) and also Atlanta for many years, I'm used to seeing jets come in for landings.
But this felt different.
So I thought "Maybe it's a military plane, and maybe there is an Air Force base nearby with a north-south runway"... I was grasping for an explanation.) But I clearly read Delta on the belly of the plane, so Hartsfield must have been its destination.
It felt so much closer than any other jet in my years of living in the Atlanta area that I went home and felt certain I'd find news stories about a Delta jet landing on an Atlanta freeway, but I haven't found any.
I can think of 2 possible answers:

Maybe I was still on the parts of 85 S and 285 W that travel west, and the plane was flying east and landing on the southernmost runway (10). And maybe this particular plane just happened to be coming in lower than any other I've ever seen but within their own expectations for minimum altitude.

But I really think that I'd already made it to the part of 285 N that travels nearly due north (shown on the left side of the map).

Maybe there are tiny stretches of 285 N/W that travel due west for a long enough time that an aircraft landing on one of the other runways would have been facing me head-on.

But I think Google Maps is really accurate, and even when zooming in, I see no part of the 285 N freeway west of the airport that would have taken me due west.

Would there be any other explanation?

You should edit the title to be a question as this is standard on yhis Q&A website

Since you only "really think" that you had made a turn to the north, I would vote for the first of your possible explanations.  There are a lot of turns just getting to the freeway, I would bet that you got twisted around and perceived the turn from S/SW to W as the turn from W to N/NW, and were actually further back that you realized.  It happens to me all the time, one turn looks very much like the next, and you realize you aren't as far along as you thought.

Two things. One, that area of I-85 and I-285 is extremely confusing, and I frequently can't tell which cardinal direction I'm facing when going through it. Second, if you did see a plane flying perpendicular to the normal flow of traffic, the plane may have been "going around" because they aborted the landing.

I don't think there is a right answer to this, all we can do is speculate.  I have seen questions closed for less...

“Yesterday a few minutes after 4:00pm ET”—did I derive the date correctly to 2020-01-04 16:00-05:00, converted to UTC as 2020-01-04 21:00Z?

I've replayed 2020-01-04 20:45Z to 2020-01-04 22:00 on FlightRadar24 and seen nothing but a completely regular flow of western traffic with active runways 27 and 28.

@MichaelHall, well, we can find the aircraft in question on flightradar. But I saw none. if I had the time corect—the question should state date, not “yesterday”, because the post is going to stay for many days and it won't be “yesterday” tomorrow anymore.

@Jan Hudec, good point, I didn't think about replaying FlightRadar24!  Your findings do confirm my hypothesis though.

@JanHudec Correct, 2020-01-04 21:00 UTC. Thanks for pointing me to that really useful site; I'd accept it as an answer! I filtered to only planes in the air and must have callsign with "DAL" since it was a Delta jet. I see DAL695 with estimated arrival of 16:16 ET, but that one flew east (not south) and did not even land on the southernmost runway (which would be closest to 285 West), so that couldn't be it because it's not flying along freeways southbound or eastbound, so I wouldn't have thought it was about to land on one. This site hasn't shown me a Delta plane tracing a freeway yet…

@MichaelHall and Jan: How strange is this? https://imgur.com/a/dAF8exK  DAL695 (N389DA) is shown as this red plane flying westbound https://www.flightradar24.com/2020-01-04/21:12/12x/DAL695/2369833d over runway 27L at 21:13 UTC and then seems to hover for a moment before immediately bouncing eastbound to land at runway 09L. That would be impossible, so I wonder if I don't know how to use this flightradar24 site.

@Ryan, when the ADS-B receiver misses a squawk from the plane, the position is estimated according to the last known velocity vector. As the data dropped just as the plane slowed down and turned off the runway, the interpolation made it to appear to overshoot the runway, and then return back when next valid position report arrived. That kind of error appears on FR24 all the time.

@JanHudec If I understand you correctly, this means that DAL695 was landing from the east (facing west) and was definitely not the plane I saw somewhere over the freeway west of Hartsfield. And FR24 didn't show any Delta planes during that time range that could have been the one. How odd.

@Ryan, indeed it wasn't and indeed I've seen no aircraft there that could match the position you describe, not just Delta ones, but none at all.

Was the aircraft's landing gear down? If not, I'd posit that you more likely saw a departure from 27R. Those often begin a turn somewhat toward the South around the time they get to 285. They aren't flying due South parallel to the Interstate, but they do turn to something like West Southwest. Perhaps the banking for the turn made its path appear more parallel to the I-285 than it really was? Granted, these generally aren't flying at especially low, though heavier ones might be somewhat low at that point (and also might look lower, but are really just bigger.)

@reirab Thanks for these thoughts. Probably the landing gear was down, because I remember thinking "It's going to land on the freeway", and I'd have far less of a chance of thinking that if it looked like it was taking off. This sure is one of those times when I wish I had a dashboard camera.

@Ryan If it was a departure (which would make the most sense for being on that side of the field at that time, due to the airport being in West flow,) another reason that a Heavy could have been relatively low is noise abatement procedures. Sometimes (including at ATL,) they pull the power back after they're comfortably airborne in order to reduce noise until they get high enough and/or far enough away from heavily-populated areas, then spool back up to normal climb power.

@reirab +1 It's cool that you know all this. But it's still not making sense to me. It seems like you're saying it could have taken off westward from 26R and then immediately made a >90-degree turn southbound (while still very low to the ground) while I was driving north on 285 N. But I don't see anything like that on flightradar24.

@Ryan I don't think it would have been > 90 degrees by the time it got to 285 (more like ~20-30 degrees,) but maybe it looked more parallel to 285 than it really was? The only aircraft I saw on FR24 at that time that were actually a reasonable approximation of parallel to 285 were all the way up at cruise altitude. But a heavy jet would make the most sense for being and appearing low, though, due to the combination of reduced power for noise abatement, longer takeoff run and slower climb due to weight, and also just looking lower than they really are because of their massive size.

@reirab I tried finding a site other than FR24 and found https://planefinder.net/flight/DAL1604/time/2020-01-04T21:15:00.000Z/speed/20 but don't see any plane in the time range that was oriented how I saw. I'm sure I look crazy by this point.

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text:
Does every device/software, developed for use in the cockpit, need to be certified?
Every avionic, developed for use in the cockpit has to complete a long and expensive certification process.
Is this only applying for avionics, installed fixed into the aircraft or also for apps like ForeFlight for example?
For example: If I had developed an app similar to ForeFlight to use on the tablet, is there anything I had to do in regards to certification or could I just sell it to pilots?

If it is permanently attached to the airplane then it needs some kind of approval—TSO, PMA, or STC. Otherwise, in Part 91 flying, you can use whatever you want—provided the PIC has determined that they do not cause interference with the radio or nav equipment. Handheld GPSs, and tablet EFB apps like ForeFlight and WingsX are not attached so they do not need approval. The exception is for Part 135 and 121 operations (charter and airline). They need to amend their op specs in order to use EFBs instead of paper.

You don't specify which category of aircraft and which legislation you refer to. For instance, in most of Europe, if your plane does not have a type certificate, then your avionics don't necessarily have to be certified. You may have to employ a few certified instruments, but they would typically be mechanical (altimeter, tachometer, compass) and don't fall in the "avionics" category. This provides you with a path to develop and test avionics at lower cost, and when ready you can put your product through the certification process.

Experimental aircraft do not need certified (TSO'd) avionics.

Based on my work experience: is it essential for normal flight? yes -> "DAL A" no -> DAL B/C/whatever depending on how much "no"

@Federico it really comes down to failure modes, and you can always go up. I've seen PFDs certifiable to level C. The Phenom 100/300 & Hondajet HA-420 GCUs were level B.

@jscarry Note that air carriers are not prohibited from approving  cockpit use of PEDs as supplements to paper charts, which is different from using PEDs as approved EFBs instead of paper charts.

@JScarry This is a common misunderstanding and is not correct. This question has come up many, many times, but the crux of the response is "show the FAR which prohibits installing avionics which are not PMA/TSO/TC/STC". Only the XPDR needs to be TSO'ed, and transmitting radios need to be FCC certified, but that's about it. The rest is between you and your mechanic.

The answer depends on the functionality and capability of the app. Some basic apps do not need to be certified, whilst more complicated ones will (either by the airline or by the regulator).
Tablets are split into 3 categories (these are the US FAA definitions):
1) Portable devices not mounted to the aircraft with read-only access to aircraft data.
2) Portable devices that can be mounted and can share information with aircraft systems.
3) Permanently mounted devices with access to the flight management computer.
There are also 3 classes of app:
A) Basic apps providing info such as aircraft manuals and procedures.
B) More complex apps providing some navigation info, weather info or exterior views of the aircraft.
C) More complex apps that can provide navigation information and can include 'own ship' position via GPS or the aircraft navigation platform.
Class 1 and 2 tablets can only host type A or B apps. Type A and B apps can be 'self certified' by the airline, providing that documentation on failure modes and usability is available. Some restrictions are in place for type B apps displaying, for example, weather info. Type C apps require regulator approval.
That's a brief summary, but it's very complex. Full details available
in this FAA document. Regulations worldwide vary, but most are broadly similar to those specified by the FAA.

What about tablets that are not permanently attached to the aircraft and which do not get data from aircraft systems?  Where do those fit into the classifications you mentioned?

@RalphJ As mentioned, that's a Category 1 tablet.

Type certified aircraft are going to have certifiable avionics approved through TSO, PMA, or TC/STC. Those avionics with software that must be certifiable will likely follow RTCA DO-178(B or C) Software Considerations in Airborne Systems and Equipment Certification.
Following from JScarry's comment:

If it is permanently attached to the airplane then it needs some kind of approval—TSO, PMA, or STC. Otherwise, in Part 91 flying, you can use whatever you want—provided the PIC has determined that they do not cause interference with the radio or nav equipment. Handheld GPSs, and tablet EFB apps like ForeFlight and WingsX are not attached so they do not need approval. The exception is for Part 135 and 121 operations (charter and airline). They need to amend their op specs in order to use EFBs instead of paper

This is a common misunderstanding and is not correct. This question has come up many, many times, but the crux of the response is "show the FAR which prohibits installing avionics which are not PMA/TSO/TC/STC". Only the XPDR needs to be TSO'ed, and radios need to be FCC certified, but that's about it. The rest is between you and your mechanic.

Good luck finding an A&P who is willing to attach a part that is not TSO, PMA, or TC/STC to a certified aircraft.

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How should these translated takeoff and climb settings be interpreted?
I am having difficulties understanding this transcript. It is originally not in English, so I was wondering if anyone could help me to make sense of this and also help me  with the correct wording in English. The training cap originally says

after the ground roll (four engine turboprop) : 12 degrees nose-up; 1500 feet; decrease power/ reduce engine thrust power (?); climb speed (?); flaps up.

I know that usually at around 1000 feet agl the pitch is lowered so I imagine that was an order to lower the pitch to 12 degrees nose up or a confirmation of that. Does that make sense? Would one lower it after the final phase of the initial climb?
Then there is the mention to decrease power. Would power be reduced at height?
I've found a document from a flight school that says power is reduced to remain in the traffic pattern, but that was after 2500 feet. And I found a skybraero page that says that after 1500 ft aal one would "reduce thrust to climb power". Would reducing thrust make more sense than reducing power? I thought they both meant the same thing. How are they different?
At the end, when (I think) he says climb speed. I found nothing about climb speed specifically... since this is said after the final phase of the initial climb out (that goes up to 1500 ft), when climb to cruise phase starts, would it make more sense if that read climb to cruise speed/ cruise climb speed (Vcc)?

It may help if you noted the model of aircraft involved, there are a number of 4 engine turboprops that may have different procedures. Some engines can't use full take-off power for more than some period of time, so the procedure is to lower the power setting.

12 degrees nose up

Since it says this comes after the ground roll, it sounds like this is the pitch you rotate to when you reach rotation speed. You then hold this pitch for the initial climb out.

1500 feet; decrease power/reduce engine thrust power

In English, generally "power" is used on propeller aircraft, and "thrust" used on jet aircraft. Other languages might not translate exactly. On a turboprop airplane, you would have power levers, which you use to select a power setting. At this point, you would decrease the power setting to a climb/continuous setting.

climb speed

This just means you will fly whatever speed you plan in climbing at. This would depend on the aircraft, and the flight. Especially larger, higher performance aircraft will have some standard "climb speed" that is used during climb, to balance ground speed and rate of climb. "Cruise climb" may be used to describe a slower climb where emphasis is placed on a higher ground speed. In smaller aircraft there may be less performance margin. This wording may also be leaving it open, as on some flights you may want best rate/angle of climb, others would use a regular climb speed.

It would be common for the takeoff power setting to be higher than the climb power setting, and also common to climb steeply to 1000 (or 1500, depending on the specific procedures of the operator) feet, then reduce pitch in order to acelerate.  As speed builds, the flaps would be retracted.  Also at about this point (either when pitch is reduced or after the flaps are fully retracted), it's common to reduce the power from the takeoff power to climb power.  In general, twelve degrees of pitch seems about right for the target attitude for the initial climb.  (In comparison, the 737 uses 15 to 20 degrees as the pitch target for the initial climb to flap retraction altitude, reduced to 10 degrees to accelerate; that a turboprop's initial climb would be slightly less steep seems reasonable.)
To "reduce thrust" and to "reduce power" are essentially synonymous, and if different writers use different terms there, it doesn't reflect anything significant.
The initial climib at 1000 or 1500 feet is typically performed at a speed slightly above takeoff speed; the climb from that point to cruise altitude is typically performed at a speed referred to as "climb speed", and then at cruise the aircraft will fly its "cruise speed" which is usually (not always -- there are exceptions, especially when using Mach Number as the reference for the higher stages of the climb) somewhat faster than the climb speed.

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Where can I find airfoil data for the Beechcraft 1900D tail?
I am creating a CAD model of Beechcraft 1900D from images for a project. So far most info is available in http://www.users.on.net/~grypen/b1900d.html except the airfoil used by the tails. I have searched extensively and wasn't able to come up with any data. I am new to this and may be unaware of popular sites for these kinds of data. Any help is much appreciated. Thanks!


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When should I notify ATC when I notice that I may have conflicting traffic?
Let us say I am flying at FL100 with heading 360 and I noticed that ATC just cleared another aircraft to climb to FL100 with heading 270, and we may have a converging path if we proceed with our current flight paths; so, if, from then on, neither aircraft has gotten any further instructions from the controller, at what point should I remind ATC that I may have conflicting traffic based on my current flight profile and the clearance given to the other aircraft? For instance, would I have to wait until ADS-B alerted me of a loss of separation between me and the other traffic (say, coming within 3 nm of each other)? Or I could report the possible conflict earlier?

Generally speaking, you should trust ATC. As a pilot, you do not have the full picture. Just because two aircraft are cleared to the same level, does not mean they are going to get anywhere near each other.
As for using TCAS or similar, this article describes an interesting incident, where one pilot decided to make a turn, based on his TCAS display, which resulted in a loss of separation. If the pilot had followed ATC instructions and not turned, there would have been 15 NM between the two aircraft. Instead, the separation was reduced to 2 NM! The problem is that a TCAS display only shows the relative position of other traffic, which makes it hard to determine the direction and speed of that traffic. For that, only a stationary radar station, like the one ATC uses, will work correctly. TCAS is NOT radar!

would I have to wait until ADS-B alerted me of a loss of separation between me and the other traffic (say, coming within 3 nm of each other)?

ADS-B cannot warn you about a "loss of separation", only a risk of collision. Your ADS-B has no idea what the separation minima are at your current location. If flying across the ocean with no radar coverage, the minimum separation could be 60NM. If flying close to an airport, within visual range of the tower, 1-2NM could (in some cases) be legal.
Please understand that ADS-B, TCAS or similar technologies are not meant to replace ATC. They only work as a secondary safety net, designed to kick in when ATC fails.
With that out of the way, if you find yourself in a situation where you are completely sure ATC has messed up and you are on a collision course with another aircraft, of course you should react. Ask ATC to verify your clearance. If you see the other aircraft, follow the rules of the air. But, to be blunt, I would have a very hard time imagining a scenario where you would notice a conflict before ATC and come up with a better solution than ATC. Don't be the guy who questions everything that is said on the frequency. If you felt like you were in an unsafe situation, call the tower (on the phone) after landing and hear their side of things before you jump to conclusions.

thanks for clarifications. The incidents you mentioned are caused due to TCAS only provide the relative position of other traffic, but thru a ADS-B system, I should be able to interpret the path of the concerning traffic, since the ADS-B could provide me the velocity, heading of another aircraft, right? Is the intruder's path drawn on a controller's display also built per the ADS-B data?

I think you will find there isn't a correct answer for this, and that answers will primarily be opinion based.  That said, my opinion is that if you have ADS-B, be patient and let ATC do their job.
Speak up eventually, of course.  Especially if you see a clear converging path on your scope.  The 3nm you suggested seems like as good a point as any to mention it.
It's a little different if you are VMC.  In that case I might wait even longer and attempt to acquire a visual first.  That way your reminder to them can consist of your assessment of the need to deviate, or declare that you can maintain visual separation.


text:
Why does a pilot bank up to 5 degrees into the operating engine following failure of the other engine?
If a multi-engine aircraft suffers an engine failure while near minimum control speed (Vmc), one of the solutions is to bank up to 5 degrees into the operating engine to increase rudder effectiveness to maintain control. Why is it up to 5 degrees? What happens if the pilot banks more than 5 degrees into the operating engine?

Closely related, maybe a dupe?

@Pondlife I don't think this is a dupe. The current question asks specifically about the reason of the 5° mentioned in this answer

The 5 degrees of bank is to create a side slip component that offsets the skewed thrust line created by the asymmetric thrust, and the rudder input made to counteract the asymmetric thrust.
You have the live engine on one wing that wants to make the airplane turn.  You apply opposite rudder to stop the turn.  With the rudder moment pushing sideways, you end up with a resultant thrust line that is offset, and the airplane proceeds forward with a lateral skew toward the dead engine even though you think you're going straight.
By banking into the live engine, bank angle makes the airplane want to side slip toward the down wing, which is in the opposite direction to the skew effect mentioned above.  The 5 degrees of bank is roughly what gives the necessary amount of side slip tendency.  Close enough in other words.
The result is that you will be flying with 5 degrees of bank, but actually proceeding straight through the air.  The skid ball will be offset into the bank because you are actually still in coordinated flight and the offset location of the ball is the true "centered" location.

You're saying in the first paragraph that the bank creates a sideslip component, but then you say in the last paragraph that you're still in coordinated flight. Doesn't coordinated flight mean that there's no sideslip? Can you clarify what you mean there?

You are in coordinated flight; the tail is lined up behind the nose in the airstream, because the sideslip tendency created by lowering the wing is offset by the skewed effective thrust line.  Imagine you're in a car on slick ice driving straight, but the wheels are crooked, making the car dogleg sideways somewhat.  If you tilted the ice surface laterally, so the car wants to slide downhill, just the right amount, the lateral sliding tendency cancels out the dogleg tendency and the car goes straight along even though the surface is slanted.  Sorta like that.  I've added a crude diagram.

On a marginally power piston twin, the 5 deg bank that is done to cancel out the sideslip induced by the offset thrust line can mean the difference between climbing and no climbing.  If you don't lower the wing, the airplane is in fact sidesliping with the wings level, basically being pushed sideways by the rudder you might say, with the resulting drag rise.

No slip is taking place; banking produces a sideways force to counteract sideforce from deflected rudder, thus eliminating any turning tendency ( heading change ) .  Generally speaking, banking does not cause or cancel sideslip.

I don't think the description is quite accurate. The bank utilizes gravity to offset the side force introduced by the rudder, freeing up sideslip so that less rudder would be required. On another note, I think the OP has a good idea about the VMCA maneuver, but was asking why 5 deg in the regulations (and not 6 or 7 deg)?

I'm suspicious of explanations involving gravity.  The rudder is used as needed to prevent sideslip (as measured by the yaw string not the slip-skid ball),  and the aircraft is banked as needed to eliminate any turning tendency (heading change) due to the deflected rudder.  That's really all there is to it.

@quietflyer That's the physics :). Gravity is the vector that opposes aerodynamic side force (from sideslip, rudder, ailerons, MFS, etc.). There's no turning since the body rates are zero at steady-state.

@quietflyer  The offset thrustline vector generated by asymmetrical thrust plus rudder does not generate a turning tendency. It simply flies in a direction that's offset several degrees from its heading, without turning.  Banking creates a counteracting sideslip TENDENCY equal to the skewed thrust vector, that aligns the direction of flight with the longitudinal axis.  You are banked, but you are going straight. If you don't bank into the dead engine and just hold enough rudder to stop a turning tendency from the thrust asymmetry, you are still slewing laterally and climb is severely degraded.

@quietflyer banking does cause sideslip.  How much depends on the weathervaning tendency of the fin.   Dihedral exploits the sideslip tendency to provide lateral stability.  Dihedral doesn't work without sideslip induced by banking.

@JohnK don't make me break out math :)

So will the math show that dihedral effect does NOT require sideslip due to bank?

@JohnK I'm not sure what you mean. What does this have to do with OEI straight flight? It's a slightly more altered version of SHSS.

Well I definitely see some sideslip when I do a steep wingover with feet off rudder pedals, and it's possible that gravity is somehow involved, but I'm still not convinced the same is true in a steady-state constant-bank situation.

When you bank the lift vector is canted and the airplane moves sideways.Because it's moving forward as it does so, moves in an arc, a turn.  The fin weathervanes the fuselage to keep the long axis aligned with the arc of the turn,after a bit of slip. If there was no fin, it would be all slip.  Anyway, when you are single engine, the skewed effective thrust line wants to move the airplane laterally into the dead engine and the lateral force of the bank cancels it out. You will be flying straight with the wings banked, unaccelerated laterally, so the ball will be straight down, 5 deg off center.

@JohnK On bank, the lift vector is not canted with respect to the aircraft body axis. The gravity, which always points down inertially, is.

@Jimmy I think in the end we are talking about the same effects.  Perhaps I'm just using the wrong terminology.

@JohnK-- re "Anyway, when you are single engine, the skewed effective thrust line wants to move the airplane laterally into the dead engine"--  I think that it is actually the sideforce from the deflected rudder that wants to move the airplane laterally into the dead engine. We bank to cancel that out.  Without banking, we'd have to apply more rudder to drag the fuselage sideways through the air to generate a lateral force component toward the good engine. Also, a steady unbalanced lateral force leads to a turn, not a slip. So I don't see the banking as actually creating a slideslip component.

But banking does allow us to bring the net horizontal force to zero-- despite the fact that the rudder is generating a sideforce component-- without dragging the fuselage sideways through the air to generate an opposing sideforce component. So we can hold heading without dragging the fuselage sideways through the air.

This actually has echoes of the question, does an aircraft automatically start slipping sideways through the air simply by virtue of the fact that it is banked, or do we need to look at other deeper nuances to understand the cause of the sidelip?  Relates to how dihedral works, etc.  Have seen erroneous ideas about this in several sources such as the well-known "Model Aircraft Aerodynamics" by Martin Simons.

It's like you were moving on a icy road that suddenly tilted to the side. You start sliding downhill, laterally that is, as you're moving forward. Didedral wants to return wings to level and the rudder wants to weathervane the plane into the offset airflow and spiral.  Dihedral and rudder/fin sizing has to balance those two characteristics out.

These figures are a regulatory baseline for sizing the ailerons, vertical stabilizer and rudder for an aircraft.  The 5° bank limit is done to minimize the load factor on the aircraft while providing a force to counteract rudder input required to maintain a coordinated flight path.
In the event of an engine failure in a non-centerline thrust twin or multi engine aircraft, the operative engine is going to create a strong yawing moment about the vertical axis of the aircraft in the direction of the dead engine.  Uncorrected, this results in a forward slip toward the side of the good engine and, when combined with the fuselage blanking airflow over the wing on the dead engine side, a rolling moment also develops about the longitudinal axis in the direction of the dead engine.  At low speeds, combined with the high drag created by the slipping condition plus a 50% loss of total available thrust from the engine failure, this can quickly snowball into a departure from controlled flight and crash.  The typical action is to apply rudder in the direction of the good engine to counteract this forward slip.  However, while the nose will be aligned with the desired flight path doing this, the actual flight path is a side slip towards the dead engine side, which creates excess drag.  The only available counter to this is to bank the airplane into the direction of the good engine to counteract the rudder force using the horizontal component of lift.  This results in a coordinated flight track parallel to the horizontal axis of the aircraft with a minimum amount of drag.
If excessive bank angle is used to do this, the vertical component of lift is diminished, requiring a greater angle of attack to be imposed upon the wings to stay aloft.  This in turn creates more induced drag.  The regulations for aircraft design of light twins, therefore, dictated that, in a worst case Vmca, directional control must be maintained with a bank angle NOT GREATER THAN 5°.
Harry Horlings, a former military test pilot and aviation consultant, published this excellent video on the nature of Vmc and what it means to the design and operation of aircraft.

This statement appears to contradict itself: " However, while the nose will be aligned with the desired flight path doing this, the actual flight path is a side slip towards the dead engine side, "  Banking does not cause or prevent sideslip.

It does prevent it if you have a dead engine and need to apply rudder force to counter the yaw due to asymmetric thrust. If you do a force diagram you will note that the airflow deflecting off the rudder causes a lateral force to be applied to the tail fin. This lateral force will cause the airplane to slip in the relative wind. A slight bank towards the good engine will direct just enough of a horizontal component of lift to counteract this rudder force and prevent a side slip.

@CarloFelicione Your above comment is good. I would suggest dropping the word "slip" since it evokes sideslip, which is not the case here. It's also not the lift per se that generates the side force, but close enough. I'm suspicious of lift vs bank as the reason for limiting to 5 deg. At even 10deg bank, the extra load factor is only 1.5% of the level flight lift.

@CarloFelicione -- re your comment above-- in your example I would say that a more detailed description of "applying rudder to counter adverse yaw" would be to say that the ideally the pilot is applying exactly enough rudder so that the plane is flying with zero sideslip as measured by a yaw string on the nose, despite the dead engine.  As you point out the rudder will still be causing a sideforce which will displace the slip-skid ball off-center.  The purpose of banking is to prevent this sideforce from causing a heading change-- not to alter the position of the yaw string or slip-skid ball.

(Continuing)-- Yes, due to the interrelationships involved, if we are simply using the rudder as a "steering" control to hold a constant heading, then it's true that too much bank would correlate w/ a slip toward the GOOD engine, and too little bank would correlate w/ a slip toward the bad engine, but the rudder is not really a heading-holding control, and looking at it this way "puts the cart before the horse" and obscures what is really going on. For more see my related answer to related question https://aviation.stackexchange.com/a/65877/34686 -- especially paragrph beginning "Now, granted"

(Continuing) A key point in all this is that fundamentally, slip (or "sideslip") is not caused by a sideways force.  An unbalanced force component causes a turn (curvature in path of travel), not a slip.   Slip can be a linear condition-- it is not necessarily related to turning and thus need not involve any unbalanced force component.  Slip is caused by not pointing in the same direction you are actually moving through the air.  Again for more see related answer to related question https://aviation.stackexchange.com/a/65877/34686  .

A failure of an engine in a twin does not create adverse yard. Adverse yaw is associated with a byproduct of the lift deferential when aileron is applied.   It has nothing to do with the asymmetrical thrust yaw from a failed engine.   In the case of a twin with an in operative engine the writer required to counter the asymmetrical thrust yaw  Will cause a flight path not in line with the longitudinal axis of the aircraft. This essentially becomes a side slip.

To maintain straight flight after one engine inoperative (let it be right hand engine), rudder input (nose left) is required to take out the yaw asymmetry from the engines. As rudder is deflected, it produces an aerodynamic side force (to the right), which, if left as is, would push the aircraft into a skid turn. This would not constitute straight flight.
To zero the side force, and to maintain level flight (ball centered), the only recourse is to utilize sideslip to generate opposite aerodynamic side force. This means a sideslip nose left in our scenario, which means even more rudder nose left. As speed is decreased, increasingly larger rudder would be required. At some threshold, rudder would be saturated and level flight would no longer be possible below this speed.
But what if we relax the requirement of level flight? What if we allow a bank angle into the live engine (bank left wing down)? In this case, we are allowing a little portion of the gravity, equal to $W\phi$ for small bank, to help out with the aerodynamic side force. Correspondingly, less sideslip and rudder would be needed. In fact, if sufficient bank angle is used (usually after a few deg), we can allow the aircraft to sideslip into the failed engine (nose right); a nose right sideslip would generate aerodynamic nose left aerodynamic yaw, further decreasing the rudder required.
By allowing banking, we can decrease the speed threshold to which the control surfaces would saturate, thus lowering the minimum control speed (Vmc).
Throughout it all, rudder generates an aerodynamic rolling moment, as does sideslip, which must be countered by roll control. As bank angle is increased, the aircraft will be less rudder limited, and more roll control limited. Under FAR 25.149 (and the old 23.149), a maximum bank angle of 5 deg is allowed for the determination of Vmc. Different aircraft will be limited differently at 5 deg bank; some may be limited by rudder, others by roll control, and still others by stall warning.
For those still not convinced, please refer to the following equations, which must hold true for steady/straight flight:
$$0=N_{engine}+qS_{ref}b_{ref}(C_{n_\beta}\beta+C_{n_{\delta r}}\delta r+C_{n_{\delta a}}\delta a+C_{n_{\delta s}}\delta s)$$
$$0=C_{l_\beta}\beta+C_{l_{\delta r}}\delta r+C_{l_{\delta a}}\delta a+C_{l_{\delta s}}\delta s$$
$$0=W\phi+qS_{ref}b_{ref}(C_{y_\beta}\beta+C_{y_{\delta r}}\delta r+C_{y_{\delta a}}\delta a+C_{y_{\delta s}}\delta s)$$
Even more additional information can be found in AC 25-7C Appendix 6.
What would happen if you fly more than 5 deg into the live engine with OEI, nothing much, unless you are flying at Vmc, which would smaller than $V_2$ and $V_{REF}$.
As to why 5 deg, and not 6, or 7 deg? My guess is that it's a rounded number that offers adequate decrease in Vmc for performance, yet not so much as to introduce large lateral acceleration and a big disparity between (a high) low weight rudder limited OEI speed and (a low) high weight rudder limited OEI speed.

The failure of an engine in a multi engine aircraft causes it to yaw towards the failed engine. To counter this, the pilot has to apply the rudder towards the live engine. This is all great as the rudder itself can keep the aircraft balanced.
Even though the aircraft looks balanced with the rudder alone, the application of the rudder puts the aircraft in a side slip. With the rudder applied the force on the vertical stabilizer acts through the aircraft CG to keep the live engine thrust from yawing the aircraft into the dead engine. This causes air flow to hit the vertical stabilizer from a side generating a side force that opposes the rudder force. This side force then puts the aircraft in a side slip towards the failed engine.
To give you an example, think of an aircraft that suffers from a right engine failure. As soon as the engine fails, the live left engine thrust yaws the aircraft into the right engine. The pilot counters this using the left rudder. The rudder creates a force on the left side of the vertical stabilizer which allows the pilot to fly the aircraft without losing control. As the rudder is applied, the nose of the aircraft points to the left causing the relative air flow to hit the aircraft on the right side. The result is a left side force which puts the aircraft in a right side slip. Look at the picture below.

https://www.boldmethod.com/blog/und/how-does-zero-sideslip-work-in-a-multi-engine-aircraft/
The reason why we put in a small bank towards the live engine is to reduce the effects of the side slip. An aircraft in a side slip creates drag which is detrimental to the performance of the aircraft particularly in a climb. When you bank into the engine, the lift component generates a side component which allows you to fly the aircraft in a controlled state with a reduced rudder deflection. The end result is a reduction in the side slip which increases the aircraft performance. This also reduces the minimum control speed of the aircraft, Vmc.

The rudder is used as needed to prevent sideslip (as measured by a yaw string not the slip-skid ball),  and the aircraft is banked as needed to eliminate any turning tendency (heading change) due to the deflected rudder.  That's really all there is to it.

The correct physical explanation of OEI straight flight (would be better if you add gravity). However, I still believe the OP is asking why the regulation states 5deg instead of another number.


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Where can I find a description of the ECAM system pages?
I'm studying up on the display formats of the System Display (SD) in a typical ECAM. Articles online have stated that it is capable of showing 12 different system pages with different info. However, I cannot find anything that goes in-depth on explaining all the system pages. Any help is appreciated.

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.

This bot is not very smart...

If your interest is more of an academic (i.e. non-operational) character you'll find plenty of information on that in https://www.smartcockpit.com/plane/AIRBUS/A320.html.
A general description of the ECAM philosophy is found under "Indicating and Recording". Then specifics on each system page are found at the specific system section, i.e. the section on hydraulics will feature the information in the hydraulics SD page.
If you're studying for a type rating, which I assume is not the case since you mention a "typical" ECAM, then your reference absolutely needs to be your current AFM/FCOM.

You may wish to take a look at www.ipadecam.co.uk  which will take you through all the screens

Please post the relevant parts of your source here. Only posting a link is less useful, especially if the link stops working.

@fooot That would be really hard in this case -- the site has an interactive graphic demo of how ECAM works.  The question is, "where can I find..." and this post provides an answer: "this site."  If the site goes away, then this stops being a good answer, but in the meantime, this answers the question directly by pointing to a very good training aid.  There simply is no option to post a few paragraphs or a couple of screenshots of the site in order to capture the value that is there.

@RalphJ This is why we don't like answers like this, even if they do ccurrently answer the question: Your answer is in another castle

@Jamiec And yet, this answer helps out the OP & any others who come here w the same question. Is it better to disallow providing this sort of assistance to those who come here seeking help?

Also, it's clear that this answer is distinguishable from the "take the awesome tutorial HERE" bad example in the "other castle" post in multiple ways: site URL is visible, what's there is described in the answer, and this is exactly what is asked for in the OP. An example of most wonderful, most informative, bestest answer ever? Not quite. But still a valid & helpful one.

@RalphJ will this answer still be helpful when that website disappear? if not, this is not an answer.

To be fair, this isn't all THAT much worse than Joao B's answer...


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How to troubleshoot a noisy aircraft engine driven hydraulic pump?
I understand that engine driven hydraulic pump noise is caused by several reasons such as cavitation,aeration or mechanical defects but how to troubleshoot this problem using a flowchart from any aircraft troubleshooting manual?

What aircraft? You need to be specific about this you cannot just ask for any aircraft or it is too broad.

I prefer Learjet 45 or airbus a320

A good mechanic can listen to a noisy pump with a stethoscope and tell whether it is cavitating, has a broken impeller blade, or bad shaft bearings. But the proof is always removal and disassembly. Once the pump is removed, then the mechanic can tell by spinning it by hand whether the bearing balls or races are pitted, and by shaking the drive shaft back and forth whether or not the bearings are beyond their wear limits.
I think the right thing for you to do is ask a real, live powerplant mechanic this question.


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Do airliners get substantially different weather briefings from the rest of us?
Southwest Airlines flights were delayed and cancelled this week due to an outage in their third-party weather provider. I understand if they have a streamlined business process for getting weather data to their pilots, but is that really the only way for them to get their en route/destination weather? Is there a reason they can't obtain and use the same type of briefing a GA pilot would use?

It would've been great if every single Southwest Pilot started calling 1-800-WX-BRIEF for a detailed telephone briefing. :)

My understanding is that pilots may not, but the dispatchers need winds aloft information to avoid sending aircraft against 150 knot headwind and to generally optimize fuel burn. And this is a lot of data, so it needs to be fed into the dispatcher's computer and if it's not coming, the dispatch system is only half-usable and then flight plans are not prepared in time.

Most GA pilots don't spend much time around 30,000 feet.  And I expect that (except for takeoff & landing, of course) most airline pilots don't care what's happening down here close to the ground :-)

Related: What is the role of the flight dispatcher? How do the dispatcher and the flight crew interact? and Do dispatchers ever update and re-send a flight plan to an aircraft in flight?

Airlines operate under the requirements of their Operations Specifications, or OpSpecs.  One of these will state what the approved sources of weather information are.  For a large airline, that weather provider will be providing weather to the pilots and to the dispatchers, ensuring that everybody has accurate & current info.  Most of the time, what you can get from any number of internet weather sites will basically match what you get from your airline, but the requirement is there so that the crews are always getting the same good info as the dispatchers.
And when you aren't getting that info from the approved source, per the OpSpec, you can't operate (unless there is a backup source that's approved).
On a clear day, it probably wouldn't much matter where you got the info -- "no alternate required, destination is landing to the west, 30 knot headwind in cruise, okay let's go."  But the system and the requirements are set up to keep things safe even when conditions aren't all benign.  And the uniform requirement is, get your weather from an approved source, and only then, you can launch.

Airlines usually use a third party (although Northwest infamously used their internal weather department) to get their weather because there's so much data to be obtained.
In the Center, we aggregate weather data from the NWS and NOAA that includes METARs, TAFs, SIGMETs, etc., as well as upper air data (GRIB format), GOES, and NEXRAD. Airlines do the same.
They typically fly longer routes than GA aircraft, at higher altitudes, and have larger fuel concerns.
It makes sense for them to suspend operations if they can't get that data.

I don't think it would make sense to suspend operations in that case—it's still cheaper to load some extra fuel than cancel the flight and have to reroute everybody. More likely the lack of the data made the dispatch system partly inoperable and they were not able to dispatch the flights in time.

Still, many airlines would suspend operations until they could get concrete data. The prospect of holding for 45 minutes over  for a thunderstorm at Denver and diverting to your alternate is less desirable than waiting on the ground at Albuquerque

It is, but checking whether there are likely to be any storms in the way is the easy part that can be managed with any internet connection. You can actually get the winds aloft that way too (there is a couple of sites that show model output including winds at ~FL300 and ~FL400), but without integration that part is effectively useless.


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What is the relationship between the design lift coefficient and the section lift coefficient of a NACA5 airfoil?
I would like to know the relationship between the design lift coefficient Cli, of a 5-digit NACA airfoil, with the expression Cli = 0.15*L according to Wikipedia. And the section lift coefficient of an airfoil.
I was doing a CFD project and was planning to compute the section lift coefficient of different airfoils, and comparing them to the L (1st digit of a 5-digit NACA airfoil) of the specific airfoil, but I don't know which is the relationship between the 2 values.

The design lift coefficient determines the camber line of the airfoil. It is chosen such that the flow hits the airfoil exactly parallel to the start of the camber line when at the design lift coefficient. For a NACA 23015 this would be a lift coefficient of 0.15 x 2 = 0.3. The index i only denotes that this was held to be the ideal lift coefficient.
At this ideal lift coefficient the highest local curvature of the airfoil contour is right at the stagnation point and the pressure distribution downstream from there is smooth without the suction peaks which occur when the high curvature of the leading edge has to be negotiated by the upper (higher lift coefficient) or lower (smaller lift coefficient) side flow.

Is this definition of design lift coefficient correct? The design lift coefficient is the lift coefficient at which the airfoil section produces the least drag. I've also seen it defined as the lift coefficient of the airfoil chosen so that it matches the airplane lift coefficient. Thanks

"The design lift coefficient at which the airfoil section produces the least drag." and "The lift coefficient of the airfoil chosen so that it matches the airplane lift coefficient."  Are both consequences of the definition of the design lift coefficient -- not its definition.
Imagine a thin flat airfoil (zero thickness) at positive angle of attack.  We know the rear stagnation point occurs at the TE because of the Kutta condition.
The forward stagnation point occurs aft of the LE on the lower surface.  The streamline immediately above the stagnation streamline will turn to flow up the airfoil, will turn 180 degrees at the LE, and back down the upper surface to the TE.  At the LE, when the flow turns 180 degrees at a point, the acceleration is infinite and a large pressure spike.
When at negative alpha, the same thing happens, but upside down.  The forward stagnation point is on the upper surface, flow races around the LE causing a big pressure spike.
Between these phenomena, there is one alpha where the oncoming streamline perfectly meets the LE, and the forward stagnation point is exactly at the LE.  In this case, there is no need for infinite acceleration and the resultant pressure spike.
For a flat plate airfoil, this happens at zero degrees alpha and at zero lift coefficient.
For a cambered airfoil, this happens at some alpha (maybe zero, maybe not) and at some (most likely non-zero) lift coefficient.
The lift coefficient where the forward stagnation point exactly meets the leading edge of the mean camber line is the ideal lift coefficient for that camber line, cli.
That is the definition.  The rest are consequences.
When you go to thick airfoils, the infinite acceleration goes away.  There will still be a substantial acceleration with a pressure peak -- but it is all finite.
Operating a thick airfoil at cli will reduce accelerations around the LE and will reduce pressure peaks.
High local velocities tend to increase the profile drag on an airfoil.  Strong pressure peaks require stronger adverse pressure gradients to traverse without separation.
Assume you want to operate an airfoil at cl=0.3.  One is a symmetrical foil (cli=0.0) and the other is cambered such that cli=0.3.  They have the same thickness distribution and the same LE radius.  Angle of attack of both airfoils is adjusted to meet cl=0.3.
The cambered airfoil has an 'easier' time achieving the desired lift.  It can do so without excess supervelocities around the LE and without a strong pressure peak.  It will most likely have the lower profile drag of the two.
Because of all this, the cli is usually located near the bottom of the drag polar for a conventional airfoil -- and near the middle of the drag bucket for a laminar flow airfoil.
If you want to operate a wing with chord distribution c(y) at a target CL with a desired load distribution (cl(y)*c(y)) (perhaps elliptical, perhaps not), you can determine the target cl(y).
This sectional lift coefficient distribution cl(y) achieves your induced drag goals balanced against your structural goals.
By choosing a spanwise distribution of airfoils that match cli to your target cl(y), you choose airfoils that will each have an 'easy' time doing their job.  They will each operate near their lowest profile drag condition.  This will tend to reduce the reduce the parasite drag of the wing near this design operating point.


text:
How can I calculate the "stability state" of air from a given set of data?

I would like to know the answer for the above question, it is about how to find whether atmosphere is stable or unstable using DALR (dry adiabatic lapse rate), SALR (saturated adiabatic lapse rate) and ELR (environmental lapse rate). The table also lists RH (relative humidity).

What have you tried so far? Where do you get stuck?

All I know is if ELR > DALR , its unstable and ELR<SALR, then its stable. I tried using DALR = 3 C / 1000 ft and SALR = 1.8 C / 1000 ft and ELR = 1.98 C/1000 ft

I'm unable to proceed from step-1

I have an answer below, but really, if you are studying for a licence (of any sort) figuring this stuff out is key to understanding how atmosphere works. Luckily it's not actually that hard when things finally click. It's simply about energy embedded in the airmass. Compare that energy to the surroundings, and if there is difference, tadaa: you get weather.

You should calculate the environmental lapse rate (ELR), not use a fixed value. In the table you presented, the ELR is:
$$ \frac{T_{alt_1} - T_{alt_2}}{alt_2-alt_1} \cdot 1000$$
Compare this calculated value to dry and moist (or saturated) adiabatic lapse rates.

If the value falls between them, the air is conditionally unstable.
If the value is larger than the dry adiabatic lapse rate, the air is
absolutely unstable.

For accurate results, you must calculate SALR, as it varies strongly with altitude. Formula is in Wikipedia article.
See Wikipedia: Lapse rate and  Skybrary: Lapse Rate for more in-depth explanation.
Caveat, the aforementioned sites explain absolute unstability differently, I have no recolletion which it actually is, and my study books are buried in the garage...

This answer is off to a good start.  These days we do not need to "dig up books" to finish it.  The dry rate is different from the saturated rate.  This fact drives a lot of local weather.

Yea I remember the difference between dry and saturated LR, and how and why they "work" with ELR, but the absolute instability stuff escapes me  ...and the garage is a mess

Well, +1 for the references.  The cumulus photo in the Wiki $makes$ $it$.  I think the "absolute instability" is a combination of temperature and humidity difference (because H2O is a lighter molecule as compared with O2 or N2, 14 vs 32 or 28).  This is why a "dry line" along side humid air acts the same as a "cold front".  Rising humid air is "stable" until the cloud base, i guess.

Moist, or saturated even more so,  air loses heat slower than dry air,  hence it is able to maintain buyoancy when rising.

From @Jpe61 references one can approximate a dry adibatic lapse rate of 3C per 1000 feet and a saturated adiabatic lapse rate of 1.5C per 1000 feet.  The question has a 3000 foot differential and "assumes a constant lapse rate ... and ignoring the effects of pressure change".
There are 3 choices:  stable, conditionally stable, and unstable
There seemed to be some confusion in the definition of ELR being lesser or greater than DALR or SALR because these lapse rates can be expressed as negative or positive numbers.
Expressing them as -9.0C/3000 feet for DALR and -4.5C/3000 feet for SALR gives:
An ELR < -9.0C is stable and > -4.5C is unstable. An ELR> -9.0C and < -4.5C is "conditionally stable".
The reason RH data is included is really a second step to determine if the temperature drop is sufficient to reach the "dew point", where clouds form. This is the "condition" of stability.
Beyond answering the test question, it is important to realize air will rise for 2 reasons: higher temperature or higher humidity than surrounding air (or a combination of the 2).  Humidity makes air light because H20 (18) is lighter than O2 (32) or N2 (28). (Helium is 4 by comparison).
Convection (updraft) can be very strong in thunderstorms because the lapse rate is much less than in dry air while the moisture is condensing.  Strong airflow (jet stream) over the top of one of these cells can also have a Coanda-like effect.  Combined with water vapor condensation in the cloud, updraft can get strong enough to form a tornado.
Aircraft are generally instructed to stay away from this type of weather by a wide margin.
However, the fair weather cumulus clouds are a delight to glider pilots.  They are sign posts to much more gentle updraft "thermals", which can be used to gain altitude.


text:
How does a subsonic jet engine intake design ensure correct air intake?
I think I understand that inlets are there to slow down the airflow and increase the pressure to stop the compressors stalling?. If this is the case, I don't understand how the nacelle or intake achieves this?
Has it got something to do with the relationship between pressure, velocity and area?
I am only looking for an explanation on jet engines which operate below the supersonic range.

Welcome to Av.SE!

Subsonic speed makes the inlet self-regulating. Add blunt edges, and the inlet works for a wide variety of engine and flight speeds, as it should.
An inlet is always a compromise. Therefore, it is quite a bit larger than what is needed in cruise in order to keep losses at low speed low. At low flight speed and full throttle the engine needs much more air than what is flowing into the inlet without the engine actively sucking air in. The flow lines converge from a much larger capture area when closing in on the inlet face and the stagnation point lies on the outer edge of the inlet lip.
Watch what happens when an airliner engine spools up on a rainy day, with puddles on the ground. When the engine reaches full speed, it starts to suck up even the water in the puddles below the intake! Some streamlines even come from behind, wrap around the inlet lip and get sucked in. With water droplets in them, they are very visible indeed.
At high speed, the capture area is much smaller than the inlet face. Only the central streamline stays straight, all others diverge away from that central streamline and now the stagnation point is on the inner edge of the inlet lip. Streamlines bordering the capture area flow around the lip and along the outside of the nacelle. This divergence of the streamlines is caused by the pressure increase ahead of the inlet, when the air is slowed down and exchanges kinetic for static energy. This even happens without friction losses, so the most efficient compression of an inlet happens ahead of it (in subsonic flow - supersonic speed removes that luxury).
The air speed at the compressor face is fairly constant at the same throttle setting, regardless of flight speed. Therefore, the pressure at the compressor face is lower than ambient at low flight speed (because air had to be accelerated) and higher at high flight speed.

for a subsonic air inlet, the optimum design is one in which the diameter of the inlet accomodates a flow rate that matches the flow rate of air through the first stage of the compressor when the plane is operating at cruise conditions. This means that there will be streamlines which terminate exactly upon the lip of the inlet at normal incidence; all streamlines interior to the lip enter the inlet and all streamlines exterior to the lip flow around the outside of the engine nacelle.


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Why do TFRs for spacecraft launches have ceilings?
When the FAA issues TFRs for spacecraft launches, such as this recent example the restrictions invariably limit their effect up to a certain altitude which never seems to exceed FL180. This is well below the operating altitude of much routine air traffic, so what's going on here?
Rockets must cross altitudes above this level to reach space and a collision with aviation traffic would be disastrous; so how is this risk mitigated if not by having pilots observe a TFR? This is in effect a followup question to "How are spacecraft and aircraft traffic coordinated?" to cover the part of the spacecraft flight after it has left the area covered by a TFR but before it has left the area liable to contain other air traffic.

TFR : Temporary Flight Restriction ... VFR : Visual Flight Rules ... IFR : Instrument Flight Rules ... NOTAM : Notice to Air Missions, "is a notice containing information essential to personnel concerned with flight operations but not known far enough in advance to be publicized by other means. It states the abnormal status of a component of the National Airspace System ( NAS ) – not the normal status."

That TFR is mainly for the general aviation flying under VFR. As you are in class A airspace when you fly above FL180 there is a good chance that you have to look out for a different NOTAM.
There is indeed the active NOTAM A2831/22 covering that launch including detailed rerouting information for dispatchers. Just search for ZMA A2831/22 in the FNS NOTAM search (don't forget to change the "Location" pulldown to "Free Text") and see for yourself.
So, in summary, most TFRs are below FL180 as they are issued for:

Temporary Flight Restrictions in the Vicinity of Disaster/Hazard Areas (Section 91.137);
Temporary Flight Restrictions in National Disaster Areas in the State of Hawaii (Section 91.138);
Emergency Air Traffic Rules (Section 91.139);
Flight Restrictions in the Proximity of the Presidential and Other Parties (Section 91.141);
Flight Limitation in the Proximity of Space Flight Operations (Section 91.143);
Management of Aircraft Operations in the Vicinity of Aerial Demonstrations and Major Sporting Events (Section 91.145); and
Special Security Instructions (Section 99.7).

(Source: AC 91-63D)
Those events rarely extend above FL180. When they do extend above FL180, as in this case, a separate NOTAM is usually issued which contains way more information that is normally only interesting for IFR operations.
Imagine you planning a local fun flight along the coast. Do you really want to wrestle with all those IFR NOTAMs during your preflight?

You're in controlled airspace when you fly below FL180 too... the absolute highest you can go in that area and still be in uncontrolled airspace is 2699' MSL, and that's only if you're 20 miles out to sea. Closer than that, controlled airspace starts a lot lower.

@randomhead I guess what Chris meant is class A airspace, so no VFR allowed above FL180.

I wasn't precise in my answer. I clarified it and changed "controlled airspace" to "class A airspace". My point is that there is no flying VFR above FL180 as @Bianfable correctly assumed.

Why are there separate categories for disaster/hazard areas and disaster/hazard areas in Hawaii, specifically?

TFRs are mostly intended for notifying recreational VFR traffic navigating on their own.  Above FL180 all aircraft are under positive control, so ATC can keep them clear of the airspace.


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How are the NEO LEAP-1A engines attached on the A320?
How many bolts hold the NEO LEAP-1A engine to the pylon at the forward mount and at the aft mount?

This page shows a schematic of the mounts for the original A320.  There are 4 bolts and 2 shear pins at the front mount, and another 4 bolts and 2 shear pins at the aft mount. In general terms, the A320neo pylon and mount system design was kept very similar to the original A320 design.  So it's a good guess that the number of bolts is the same, although I was not able to find any source to confirm that.

Thanks Daniel. Much appreciate your efforts to find the answer.

...though it looks like the diagrams are for the older V2500 engines.


text:
Are steam gauges more reliable than glass panels?
I have always had a deep appreciation for technology and the future and I encourage that technology should move us forward whether that be in aviation terms or otherwise but one question has been on my mind for a very long time and that is why we have computerized gauges and not the old fashioned ones with the gauge hand behind glass. In the event that a plane loses power, wouldn't it be better to have physical gauges because if the plane loses power, those critical gauges on a screen will disappear. Is it just me or does it not make a huge difference? I suppose an old-fashioned glass gauge will stop too, just the same as a glass cockpit gauge?
Thanks. This is a question I have always wanted to ask someone who has the expertise to answer it.

Thats a really long way to ask the question, "Are steam gauges more reliable than glass panels?"

Very good question! I have a feeling the way this developed was something like: „Hey, look, using digital displays we can condense the instruments, make them cheaper and easier to repair, and lose the flight engineer. Let’s add an independent power supply to make sure at least some data is displayed for enough time to land in case everything fails, so we’re not worse off than on those old-fashioned steam gauges we‘re ditching ...“

Steam gauge instruments on jets still require electrical power obviously for radio nav instruments, but also to keep the gyroscopes spinning.

AFAIK, there are no gauges on airplanes that use steam.

@jamesqf: Unsure if you're pedantic or just not familiar with the term:  Round, analog gauges are frequently called "steam" gauges for their similarity to actual steam-gauges; not because there is any actual steam involved (there isn't).

@abelenky: No, I'm not familiar with the term, and I doubt many people are.  If you mean analog gauges, why don't you say analog gauges?  It's only one extra letter :-)

https://aviation.stackexchange.com/questions/3577/where-does-the-term-steam-cockpit-come-from
http://www.ifr-magazine.com/issues/1_7/features/Steam-Gauges-Are-Safer_65-1.html
https://www.flyingmag.com/technique/i-learned-about-flying/glass-cockpits-steam-gauges
Analog is three syllables; Steam is one.

Most aircraft using electronic displays have double (and sometimes triple) redundant power supplies to keep the electronics alive in cases of power failure. The mechanical gauge panels of the not so distant past did not have these same levels of redundancy built in.
Beyond power considerations, glass panels also contain features like weather radar, collision avoidance, and navigation that further increase the reliability and safety of the system as a whole.
That said, it's fair to say that glass panels are more reliable. The funny thing is, critical mechanical gauges are still included on most panels as last resort backups.

Many mechanical gauges do not require power (altimeter, airspeed indicator, vertical speed indicator) and those that do (attitude indicator) often have a backup in larger aircraft.

@SteveKuo On jets, mechanical VSIs are largely gone due to TCAS. Primary altitude and airspeed are really electronic because an air data computer does calibration. They were ADC-based since at least the 727.

@user71659:  I'm really trying to understand: "Vertical Speed Indicators are largely gone due to Traffic Collision Avoidance Systems"?? Those two don't seem to be related?  Are you referring to some other acronym?

Yea, I think the disappearance of mechanical VSI's has very little to do with TCAS. It's more likely that they were eliminated simply because the vertical speed is shown right next to the speed tape on just about every glass panel out there. Even systems without TCAS display vertical speed in this way, thus there is no need to crowd the instrument panel with a mechanical VSI.

@abelenky My statement is correct as is. Just look at picture of a 757/767 cockpit. TCAS requires a red zone/green zone indicator on the VSI (called the RA display), and a traffic display (TA display). Most retrofits integrated this into a single digital VSI because it was the easiest. The alternative retrofit was a needle VSI with LED scale and traffic being displayed on the radar display, but that requires significant mods to the radar side (ability to draw symbols, override).

Most integrated flight displays – the so-called glass cockpit – contain multiple redundancies both on the forms of computer power, and electrical power sources to keep them operating in the event of an emergency or other electrical problems.
As an example, I obtained my multi engine add-on in a Diamond DA-42 TwinStar airplane equipped with a Garmin G1000 glass cockpit.   The aircraft’s electrical system uses a primary 24 V battery which is supplies power to all the electrical buses in the aircraft.  It is also charged by two 24V, 60 amp alternators, one on each engine, which supply power to their own electrical bus and the battery bus.   The main electrical busses supply power to the avionics power bus and both alternators can feed this bus as well.   In the event of a total electrical failure to both the battery and alternator power buses, there is an emergency battery available to power the avionics for at least 30 minutes of continued operation.
Many aircraft carry separate back up EFIS displays, each with its own emergency power supply for additional redundancy.
If you compare this with the typical systems an twin aircraft with a  conventional cockpit, most of the gyroscopic instruments are powered by a pair of vacuum pumps, one on each engine.   The turn and bank indicator is powered by the aircraft’s electrical system.   Lose both of the vacuum pumps, and you’re on partial panel.
In comparing these two systems you would have to look at the probability of an all out failure for both systems. The new glass cockpits have been just as reliable, if not more reliable, then conventional steam gauges and far more functional in the event of a failure.

Everyone is concentrating on the power supply side, but it's not the whole story. The display itself, as a unit, may fail. And it doesn't have to be the screen: failure of a single panel button or knob may block certain functionality, or even many of them. This is inherently more dangerous than a failure of any one steam gauge, but this is, of course, offset by always having at least two glass displays, each being able to take the most important functions of both.

Glass panels are more reliable and accurate in every way over steam gauges.
When it comes to gauges, the most accurate and reliable ones are found in industrial applications like steam plants or ships because weight was not an important factor.
On an application like an airplane or a car, weight is an important consideration. Obviously gauges on an airplane are accurate, but not nearly as accurate as they are in other applications. A 50 year-old altimeter might be accurate within 10 feet, which is fine, just a little jarring when landing.
With the advancement of technology, glass panels have a definite weight advantage over gauges and can have a precision, accuracy and reliability beyond a need to display. It depends on the sensor (does anyone need their air speed to the seventh decimal point?). Glass panels can self-test to determine the accuracy of the readings and alert you to the need to replace a sensor, but a gauge cannot do that.

0.000 000 5 knots is about 0.000 26 mm/s. I very, very, very strongly doubt that even top of the line aviation instruments are that accurate.

@MichaelKjörling I am describing both. Electronic gauges can be accurate and precise. But the precision is not of much benefit as you point out. It's just a capability beyond an analog gauge. Perhaps I need to rewrite the answer, since it's been edited once.

@ymb1 Just for clarification since it's been edited now: my comment was in response to gwally's discussion on accuracy and the (rhetorical) question "does anyone need their air speed to the seventh decimal point". It's not meaningful to display (say) an airspeed indication to seven decimal places if the instrument is not that accurate (by which I mean what your linked Wikipedia section refers to as "trueness") in the first place, even if the decimals were somehow useful. It's like how people convert a quantity from one unit to another and in the process go from three significant digits to ten.

@MichaelKjörling To go along with your accuracy argument, a mechanical altimeter might only be accurate within +/- 10 ft under a short altimeter range (say 1000 ft to 5000 ft), and upwards of 200 ft off at 35,000 ft. A digital system can employ various calibration techniques to be much more accurate. Precision for altitude is really capped at 1 ft, and even the displays only show gradations in increments of 20 ft


text:
How to decode ARINC 429 data?
I am looking for some help regarding decoding ARINC 429 data.
I have a CSV document that contains sequentially stored ARINC 429 labels.
My overall goal is to be able to scan for 4 specific labels and only decode those frames (data field, label field, and ssm field) and append them to the document.
Does ARINC 429 randomly come in different data formats (i.e. BCD, BNR, Discrete Bits, etc)?
Since I am looking for specific labels, is it okay to only have it set to decode BNR format or do I need to be able to identify and decode all the data formats?

This is a bit off topic for this exchange but you may want to check out this project.

Your question seems a bit off topic since it's more related to programming. If you don't find what you are looking for, try asking it on a programming exchange instead.

I think it's on topic as the actual question is about Aviation data, not the programming, that's just background info.

Do you have a specific list of labels you are trying to decode?

Arinc 429 is not about aviation? Isn't it a data bus found in......aircraft?

Even with the edit you propose your question is still unclear. You want help to decode data. You seem to have some knowledge of Arinc 429. What is preventing you to identify in your data flow the labels you're interesting in? We're not going to provide you pseudocode, but we can explain the protocol if you come with a specific problem about the data flow in your CSV sample. Be specific, else no answer can be provided.

@mins, I proposed the edit to remove references to Python in the question. So far my answer to the question seems to have helped the OP. The question really revolves around understanding the structure of the ARINC 429 word.

@selectstriker2: I wrote this comment while in the review queue, I didn't see that it was your edit, but voted to reopen as the edit was a great improvement, now that I read your answer and OP's comments, I think this is going to be solved.

Avionics systems niche in the world of computer software. I'd guess the average pilot would know more about Arinc encoding than the average programmer.

ARINC 429 is a defined standard that not only describes the packet structure (32 bits) but also the data formats for different labels depending on the equipment ID. To know how to properly decode an ARINC 429 label you need to know the Equipment ID, which can then be used to look up the meaning of a specific label.
The manufacturer of the device that the data came from may also provide the definition of any non-standard/custom labels
If you know which labels you are looking for you don't have to decode the entire label, you just need to inspect the bits 8-1 to determine the label.
In general though, the ARINC 429 specification provides a guideline for labels:

Binary coded decimal (BCD) sub-group 001 to 067, 125, 165, 170, 200, 201,230 to 237.
Binary (BNR) subgroup 070 to 124, 126 to 144, 150 to 154, 162 to 164, 166, 167, 171 to 177, 202 to 227, 240 to
  257, 262 to 265, 267, 310 to 347, 360 to 376.
Mixed BCD and BNR subgroup 260, 261
Discretes subgroup 145 to 147, 270 to 276
Maintenance and discrete data subgroup 155, 156
Maintenance data subgroup 157 to 161, 350 to 354
Test word subgroup 266, 277
Application dependent subgroups 300 to 307
Acknowledgement subgroup 355
Maintenance ISO #5 subgroup 356
ISO #5 message subgroup 357

Ohh, I see now! I currently have 2 of the 4 labels I need (204 & 210), but still looking for the other 2 (Outside Air Temperature & Barometric Pressure). So assuming I get the other two between 202 and 227, I wouldn't need to identify what format it is, I could go straight to translating two's compliment (BNR) to decimal?

Also, sorry if this is a silly question, but if I end up getting a label of 376, how would that be transmitted if the bnr representation is 9 bits long? (something like 0010000111)

do you know what equipment ID the device you are receiving data from is using?

Yes, the devices for the individual (i think), I managed to find a document with a bunch of labels w/ eqpt ids. The eqpt id I have for 210 is 006 and 204 is 060. another (possibly silly) question, are equipment ids unique to the air data computer?
edit: nevermind, I think I answered my own question based on what I just read. So yes, the eqpt id is unique to the avionics device. My next step should be to acquire the eqpt id of the air data computer I will be receiving from, correct? how can I go about that? I should note that the air data computer is not exactly accessible to me at this time

The Equipment ID describes what type of transmitting computer. ID 006 is Air Data System and ID 060 is GNSS Navigation Unit. For ID 006 - label 210 is True Airspeed in Knots, 204 is Baro Corrected Altitude in feet.

Ohh I see, so it is not device specific, but rather it is the type of device. Do you mean something like a serial # that specifically identifies the make and model of the air data computer?

Can I get back to you on that? My boss would be the one with that information, but he is out of the office for a while... Would it be possible for me to go any further on figuring out how to decode this arinc429 frames until I can get the make and model of the air data computer?

Sure, I would recommend taking a look at this brief overview first, should help identify which bits are assigned to different parts of the ARINC 429 word: https://www.aim-online.com/pdf/OVIEW429.PDF

Your company might also consider buying the ARINC 429 standard document, which defines most of the standard labels used and quite a few examples to help with encoding/decoding - http://www.aeec-amc-fsemc.com/cf/store/catalog_detail.cfm?item_id=1940

I read through the overview and I will bring up the full standard document with my boss when he returns. Does the full standard document contain everything about ARINC 429 protocol? such as label codes w/ eqpt IDs, methods that can be used to decode and encode frames, etc?

Also, I noticed it mentioned about a data type that uses 2-way communication... when I was reading before, I read that ARINC 429 uses simplex communication (unidirectional), so how would it go about 2-way communication?? When trying to acquire data, would an air data computer automatically transmit once powered or would it require a request code? when I found it used simplex communication, I automatically assumed that it would transmit once powered because I can't send a trigger command back...

ARINC 429 is one way only, so for 2-way communication it takes two ARINC 429 serial connections - 1 in and 1 out. Think of it as a streaming interface. The transmitter continually sends out data without knowing who is receiving it.

Ohh okay, so then I was correct to assume that once it powers on, it will automatically begin transmitting data, but as you said, it will not know who is receiving it, it is just transmitting? So I re-read the overview and wow did it ever help with my understanding of the protocol. I understand the labels a lot clearer now. For labels, I now know that the label codes are octal values, not decimal and are sent in 3 sets of octals in binary (i.e. Label 210 is sent as 100-001-10).

The only parts I still have a little confusion over are the sections discussing Williamsburg/Buckhorn Protocol (WBP) and BNR Data. For BNR Data, can you explain to me the scale factor? I wasn't able to grasp how the bits were decreasing fractions of the scale factor. For WBP, is it only implemented when the data bits exceed 21? does it essentially pack a block of words with words containing the regular Data words with the SOT, GFI, EOT, etc words? Would I need to concern myself with determining how to separate the blocks into their words and identify which is a data word and which is not?

I doubt you will need to worry about WBP, as it is used for file transfers rather than typical ARINC 429 data.

For scale factor, take a look at my answer here: https://aviation.stackexchange.com/a/37485/17732

Sorry, I'm still a bit confused by the explanations of the scale factor in the link you sent. From the documentation you linked and a video I watched, I got the idea that to identify the value in the data field would be decoded by identifying the sig bits in the field (the 1's) and multiplying the associated scale factor of that index (1/2, 1/4, 1/8, etc) by the max possible value of the identified label. In addition, bit 29 is the sign bit. Is that the correct understanding?

Out of the 32 bits ARINC 429 word, the label is coded by bits 1 to 8 as follows: bits 1 to 3 for the least weighted octal digit, bits 4 to 6 for the next higher, and bits  7 & 8 for the most weighted octal digits.
So the label is decoded in base 8 and not base 10, here an example.
For instance given the following binary label:
11100110
The 3 bits 110 are decoded 0+2+4=6
The 3 bits 100 are decoded 0+0+4=4
The two digits 11 are decoded 1+2=3
The label is therefore 346
Note: If all bits are set to 1, the greatest label is therefore 377.
Of course you may transmit more than 377 parameters, because similar parameters from different sources or sensors for instance AOA left, AOA right, do have the same label. To distinguish similar parameters having different sources, two additional bits (SDI - Source Destination Identifier) are transmitted to define the source.


text:
What is the available range for DME, and why do we need EFC?
How far from the station can a DME signal be picked up, and why do we need EFC?

Did you mean "How far can DME signal go from the station?"  Better question would be, "How far away from a station (transmitter) can be DME be received?"
I can't answer, I took out my DME receiver back in 1996 when I put in GPS.
What do you mean by EFC?

"One question per question," please.  These are two unrelated questions, and should be asked as such. Each one appears to be a reasonable question on its own.

Thank you so much for you guys comments!


text:
What is the difference between BHP (brake horsepower) and HP (horsepower)?
So far as I know, brake horsepower (BHP) and horsepower (HP) are the same.  But some books tell me that the two concepts are totally different.  What is the difference between BHP and HP?

For machinery that involves power transmission via shafts and gears, they are the same- and the actual term is brake (not "break") horsepower, because the apparatus used to measure the horsepower output of a machine is called a brake dynamometer.
For turbomachinery like jet engines where thrust is generated by movement of air, the concept of brake horsepower can't be applied- except in those cases where the turbine drives a propeller on a shaft. In these cases, the turbine's output shaft can be attached to a dynamometer and its horsepower output measured- and the result is quoted as shaft horsepower.

A horsepower is simply a measure of power, equivalent to 745 watts. The difference between types of horsepower measures in on a vehicle depend on if you include losses from the transmission.
For a turboprop or turboshaft engine, brake horsepower is the power at the output of the turbine, before any transmission. It is called brake horsepower after the brake dynamometer that is used to measure it. Essentially they are some form of brake that absorbs energy from a spinning shaft.
Shaft horsepower is the power delivered to the propeller or rotor shaft. That includes power losses from the transmission.
Equivalent shaft horsepower adds on additional thrust from the exhaust gases.
One more measure only applicable to cars is rear wheel horsepower. Measured at drive wheels, this includes power losses from the transfer (if applicable), differential, driveshafts, and tires.


text:
What's the cabin pressure of a Bombardier CRJ-900?
In terms of altitude (6000, 7000, 8000, etc), what is the cabin pressure in a CRJ-900?

I could be completely mistaken, but isn't that usually set by the pilot? You might want to compare e.g. How does an airliner deal with cabin altitude at an airport above 8000 ft?

The answer to this question (as asked) will depend on two factors: how the pressurization system is operated, and at what pressure altitude the aircraft is. In its current state the question is unanswerable. I think the question might be better asked along the lines of: "what is the cabin altitude at maximum differential pressure and FLXYZ".

At max differential it's 8000 ft.  The controller operates on an automatic program schedule that lets the cabin pre-pressurize to about 300 ft below field elevation at the start of the takeoff to minimize the initial pressure drop sensation following liftoff.  It then lets the cabin altitude rise 500-800 fpm, about 1/4 of the airplane's actual climb rate, until it gets to the max differential of just over 8 psi, which gives 8000 ft in the cabin at 41000 ft.


text:
Why do airlines follow these routes between Barcelona and Düsseldorf?
In the wake of GermanWings' terrible incident of flight 9525, I have noticed that the crash location is quite far from the shortest route connecting the two airports.
I then went on FlightRadar24's website and checked that this is the normal route for this flight (and that there is an alternate one that reaches equally far on the other side):

Other companies that fly the same route (Vueling 1894, Airberlin 8947) follow the same flight paths.
Why is this? What limitations are in place that prevent these aircraft from flying over Lyon (and "nearby" region)?

Large parts of France's airspace are designated military training areas. If there is military flight training going on, the choice of routing over France is very limited. The only option is to fly around these areas, which  results in the routes you found.
Higher altitude route map of France courtesy of EUROCONTROL[PDF, 8MB]
Since the PDF seems to be giving problems on some computers, I've combined several screenshots and drew the approximate route of your westerly flight (in yellow), the planned flight of GWI 9525 (blue) and Temporary Segregated Airspaces (TSA's) in red onto the map. The crash location of GWI 9525 is marked by a red/yellow star.

Note that not all the TSA's in the map are actively used at all times and that in some cases crossing is allowed but altitude restrictions apply.
In addition to active TSA's there are other considerations that come into play such as weather and airway capacity & demand. If you have to wait 15 minutes longer for a slot on the shortest route, you might as well fly a slightly longer route if that allows you to depart earlier.

It is possible that route segments through the LF-R or LF-D airspaces are only available during certain times, mostly at night. You would have to go through France's AIP to figure it out in detail.

I was on a flight once that went straight between BCN and DUS (or FRA?). I happened to be in the cockpit. The pilots were surprised about the route they got as normally on weekdays they have to fly the dogleg.

This answer got cited by the Swiss news paper Tagesanzeiger: http://www.tagesanzeiger.ch/panorama/vermischtes/Stimmenrekorder-wurde-bei-Absturz-beschaedigt/story/25279633

While DeltaLima's answer is very good, it is also due to Air Traffic Control limitations-- you want to spread out where the traffic is across the whole airspace, so routes may dogleg left or right at one point to try to keep traffic flow good (also helps separate aircraft going in opposite directions, but altitude does that too).
Keep in mind that the arrival patterns into certain airports have defined entry points and are designed to line up the plane in a certain way for the airport, and flights may have to go slightly out of their way to join the arrival.


text:
Why would pilots "call the company" after an aborted takeoff?
I was skimming through the RealATC videos on YouTube today and watched this one. In it there are two flights taking off from Chicago-Midway (KMDW); Delta flight DAL1328/DL1328 and SouthWest's SWA3828/WN3828. They are each taking off from crossing runways (4R and 31C). For some reason, even though SWA3828 is given clearance to take off, the Delta flight also starts its own rollout.
After the call to abort is given, both aircraft abort takeoff and are able to then safely leave their respective runways in order to taxi back to the proper starting point. One of the aircraft states it will need a few minutes to check for heated brakes. Then both aircraft state they will need to "call company" for some unspecified reason.
Why would both planes need to call company? My guess is to recalculate fuel used to determine if they will have enough fuel to continue the trip. Why else might they need to do so?

Likely to have the company to recalculate some performance data / fuel. If the brakes are hot, the gear may need to remain extended for a while after take-off for cooling purposes. This affects climb performance and fuel consumption.

After the RTO they may have had less than release fuel and needed a dispatch release amendment to be able to depart.

More about the incident

From the captain's standpoint, apart from the technical reasons in the answers posted thus far, think of it this way: If something has happened, whether it be an RTO or whatever, that is likely to cause your company to be notified, if at all possible, you want to be the one calling calling the company. That's better than them having to contact you. This is especially true if whatever it was is going to involve your chief pilot.

While many of the answers shared "may" be partially correct, there is a compliance issue that requires the "call company" to occur.
The Dispatch Release (the document that permits the flight to be operated) under Part 121 is valid for one operation.  When the aircraft took the runway for the purpose of takeoff and then aborted that takeoff, and then left the runway surface, that Dispatch Release is no longer valid, and would require either a revalidation or issuance of a new Dispatch Release.

Almost certainly refers to calling station ops in order to get a gate. After a rejected takeoff, even at fairly low speed, it's common to need to let the brakes cool, and if they aren't so hot as to be dangerous, cooling at the gate is preferred -- allows passengers a chance to get off & use a real restroom, buy food, etc.  Since Ops has the picture of what gates won't be needed for the next XX minutes, the crew calls them to determine where they need to go next.